1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a recording medium thereof.
2. Description of the Related Art
Owing to the advances in image input/output technology in recent years, more images are provided with higher definition and the demand for technology enabling compression/decompression (encoding/decoding) of high definition images is growing.
As an encoding method for satisfying such a demand, there is JPEG 2000 that processes a high definition image by dividing the image into small units, and decodes the image that is compressed in a high compression rate by using wavelet transformation as a frequency transformation method. In JPEG 2000, an image is divided into non-overlapping rectangular regions (tiles), in which a single tile is as a basic unit for performing a compression/decompression process. This method of dividing an image into non-overlapping tiles and performing a compression/decompression is effective for using less memory space and accelerating process speed. This method, however, has a problem of creating distortion (discontinuity) in the boundary of the tiles for a decompressed code data (code stream) that is compressed in a high compression rate.
The distortion in the boundary of the tiles may appear to be similar to block distortion in discrete cosine transformation (DCT). As a method for removing the block distortion of DCT, there is a method of adaptively applying a low pass filter to a pixel value(s) in accordance with the direction of the block boundary and edge degree (For example, Japanese Laid-Open Patent Application Nos. 5-316361 and 9-307855, or Japanese Registered Patent No. 2839987). As a method for reducing tile boundary distortion for JPEG 2000, there is a method of applying a low pass filter to a pixel value(s) in the vicinity of the tile boundary of a target decompression image (For example, Japanese Laid-Open Patent Application Nos. 2004-40252, 2004-40670, 2004-46583). Furthermore, as a method for reducing the tile boundary distortion for a dynamic picture image(s) having each frame compressed with JPEG 2000, there is a method of controlling the strength for applying a low pass filter to a pixel value(s) in the vicinity of the tile boundary in accordance with, for example, frame rate.
Regardless of how adaptively the low pass filters are applied, these methods using the low pass filter face a tradeoff problem between smoothing of distortion itself (anticipated effect) and smoothing of real edges (unanticipated effect). For example, with respect to the methods disclosed in Japanese Laid-Open Patent Application Nos. 5-316361 and 9-307855, or Japanese Registered Patent No. 2839987, although the distortion at the block boundary is smoothed by adaptively applying a low pass filter to the pixel value in accordance with the direction of the block boundary and the edge degree, these methods are, at the same time, designed to avoid smoothing of real edges situated at the block boundary.
Meanwhile, there is another method of reducing tile boundary distortion on a wavelet coefficient space with an image compression/decompression system using a wavelet transformation (See Japanese Laid-Open Patent Application No. 2001-257596).
This method of reducing tile boundary distortion on a wavelet coefficient space (in a broader term, frequency coefficient space) shown in Japanese Laid-Open Patent Application No. 2001-257596 is expected to obtain better results compared to applying a low pass filter on an image space. However, this method tends to require a relatively long process time. That is, in a case of processing encoded data subject to hierarchical frequency transformation, such as data encoded with JPEG 2000, the process time tends to increase by repetitively applying a prescribed process to plural hierarchies (levels) for reducing tile boundary distortion. In a case of processing moving images, shortening the time for reducing tile boundary distortion is one important aspect since a decoding process of a frame(s) is to be completed within a limited frame period.